1. Field of the Invention
The present invention relates, generally, to fuses. More particularly, the present invention relates to fuses having indicators that visually change when the fuse blows.
2. Description of the Prior Art
Known fuses exist that provide dual elements, so that the fuses blow if a short circuit occurs or if a current overload condition occurs. One example of such a fuse includes a short circuit element in series with a current overload or time delay element. The combination of fuse elements electrically communicates with a pair of terminals, which respectively electrically communicate with a pair of electrically conductive rounded end caps. The end caps of the fuse typically snap-fit into well-known fuse clips.
The short circuit element typically has a number of rows of slots in the metal or copper element. These rows of slots are commonly called bridges. A single, longer slot in the metal element exists between two of the bridges. The open area of the longer slot plus the open areas of the nearby bridge slots create an area around the longer slot having the highest electrical resistance on the metal element. When a short circuit occurs, the fuse typically blows at this high resistance area. The longer slot is therefore preferably placed near the middle of the short circuit element, so that if an arc occurs when the fuse blows, the arc must travel a longer distance along the blown element pieces to reach the terminals. Since the short circuit element is typically copper or copper alloy, the short circuit element does not melt due to an overload of current running through the element for a period of time.
The current overload or time delay element typically consists of a metal or metal alloy, such as lead-tin solder, which melts and interrupts current flow when a predetermined overload current flows through the element for a given period of time. The current overload or time delay element does not include a single area of high electrical resistance and therefore does not blow, or open, upon a short circuit. In this manner, the dual element fuses protect electrical components in a circuit system both from short circuits and from drawing an overload or damaging amount of current.
Known fuses also exist that provide blown fuse indication. Some known fuses provide a spring loaded mechanism, wherein the blown fuse triggers a spring that moves a plunger to a more visible location. Other fuse indicators provide a circuit in parallel to the fuse element circuit, wherein a conductive, yet highly resistive substance connects a light emitting diode (LED) or lamp in parallel with the fuse element circuit. Normally, virtually all the current flows through the fuse circuit such that the little amount of current that does travel through the highly resistive substance does not illuminate the LED. When the fuse blows, the current is forced through the highly resistive substance, illuminates the LED and thereby provides blown fuse indication.
Littelfuse, Inc., the assignee of the present invention, provides a blown fuse indicator which has a clear or transparent plastic lens that makes an internal fluorescein coated indicator coil visible to an operator. The blown fuse indicator provides a circuit that is in parallel with a fuse element. The resistance of the indicator coil is substantially higher than the resistance through the fuse element, so that current normally travels through the fuse element. When the fuse element melts, the main circuit opens, and current shunts through the resistive indicator coil, causing the coil to heat up, which vaporizes the fluorescein into a colored gas. The colored gas collects on the interior of the transparent plastic lens and provides blown fuse indication.
Another type of blown fuse indicator includes a clear or transparent plastic lens that makes an internal ball of white “gun cotton” visible. Gun cotton ignites and disappears when subjected to flames, sparks or temperatures of about 280° F. (138° C.). An igniter wire, which is in parallel with a fuse element, runs through the gun cotton. A black background exists behind the gun cotton, which is normally not visible to the operator. When the fuse element melts, current shunts through the igniter wire, and the wire heats to a temperature above the ignition temperature of the gun cotton. The gun cotton bums away, exposing the black background and providing blown fuse indication.
A further type of blown fuse indicator includes a flexible label attached to the exterior of the fuse body. The label has a colored, conductive layer fixed to the outside of the fuse body, which is connected in parallel with a fuse element. A temperature responsive layer is fixed to the outside of the semi-conductive colored background and normally blocks the operator from seeing the background. The resistance in the conductive layer is substantially higher than that of the fuse element. In normal operation, most of the current runs through the fuse element, and the small amount of current that runs through the conductive layer does not produce enough heat to raise the temperature of the responsive layer above its transition temperature. If the current in the circuit exceeds the amperage permitted by the fuse element, the fuse blows and allows current to shunt though the conductive layer. The conductive layer heats the temperature responsive layer above its transition temperature, whereby the responsive layer changes to a generally transparent state and permits the colored, conductive layer to become visible.
Each of the known blown fuse indicators provides a visual response to a blown fuse. None of the indicators, however, differentiate between a short circuit failure or an overcurrent situation. The purpose of the indicators is to provide information to the operator. The known indicators disclose the status of the circuit; i.e., is the circuit drawing a safe amount of current or not. If a circuit blows, it is also desirable for an operator to know why, so that the operator can diagnose the problem rather than simply replace the fuse. The known indicators do not provide such a tool. A need therefore exists to provide a diagnostic blown fuse indicator, which is adapted to operate with a multiple element fuse, which provides information about the cause of a blown fuse.